Parathyroid hormone (PTH) is well known for its anabolic actions in bone and is an effective therapeutic agent for the treatment of osteoporosis and localized osseous defects. Despite its clinical efficacy, its mechanisms of action are complex and elusive in particular in regard to its potential for regenerative approaches and wound healing. A key aspect of the complexity is that PTH anabolic actions are not the simple activation of osteoblasts, but require accessory cells in an intricate temporal and spatial organization. Data from the previous project award strongly support the ability of PTH to act on bone in an indirect manner through myeloid cells in the bone marrow microenvironment. The overall hypothesis of this project is that PTH acts via macrophage engulfment of apoptotic osteoblasts to produce coupling factors that recruit mesenchymal stem cells to form new bone. The specific phagocytosis of apoptotic cells is termed efferocytosis and is emerging as an integral process in tissue homeostasis, inflammation, autoimmunity and cancer, yet virtually nothing has been investigated regarding its role in the bone microenvironment. On a daily basis there are billions of cells undergoing apoptosis in the human body. Exposure of adjacent cells to the contents of dead and dying cells results in harmful inflammatory responses, yet when macrophages engulf apoptotic cells they release anti-inflammatory factors such as TGF? that are instrumental in recruiting mesenchymal stem cells. Macrophages utilize distinct receptor signaling pathways to identify apoptotic cells for efferocytosis. Such identification and signaling events represent intriguing and focused new pharmacologic targets to benefit bone regenerative strategies. Three specific aims will dissect the ability of apoptotic osteoblasts to recruit phagocytic myeloid cells, the dependence of PTH anabolic actions on macrophage phagocytosis, the specific mediators on apoptotic and myeloid cells that facilitate efferocytosis and how these link to anabolic actions of PTH in bone. This proposal will delineate these pathways relative to osteoblast apoptosis and the support of new bone formation. This is a novel new coupling paradigm in bone - macrophages - efferyocytosing apoptotic osteoblasts - producing mesenchymal stem cell recruitment factors. Such a paradigm demonstrates a significant new step in the cell biologic processes in bone as well as robust potential for new avenues of therapeutic advantage for metabolic and inflammatory-mediated bone disease.